A dead whitefish floats belly up near in the Yellowstone River in Livingston, Montana.

A 183-mile stretch of the Yellowstone River in Montana has been closed indefinitely after tens of thousands of fish have turned up dead in recent days, victims of a parasite that continues to spread.

The massive die-off has proved particularly devastating for native mountain whitefish. However, reports have emerged that some Rainbow and Yellowstone cutthroat trout — which are crucial to the state's sport fishing industry — have been affected.

The parasite is not native to the area, so it was likely introduced via a contaminated boat or fishing equipment, or possibly by birds from another waterway.

"A threat to the health of Montana's fish populations is a threat to Montana's entire outdoor economy and the tens of thousands of jobs it sustains," Gov. Steve Bullock said in a statement this weekend. "Our state cannot afford this infectious disease to spread to other streams and rivers and it's my responsibility to do everything we can to stop this threat in its tracks and protect Montana jobs and livelihoods."

Garrett Cheen / AP

The closure could last for months as thousands of dead fish turn up on river banks or bobbing in the water. Tens of thousands more are estimated to have sunk to the bottom of waterways.

The river closure is part of an effort to stop the parasite, which leads to a fatal kidney disease, from spreading and crippling the state's outdoor fisheries and tourism industries, although officials acknowledged there would likely be an economic hit regardless.

"We recognize that this decision will have a significant impact on many people. However, we must act to protect this public resource for present and future generations," state fish and wildlife director Jeff Hagener said in a statement.

The parasite poses no known risk to human health or that of other animals, however, in fish, there is 20% to 100% mortality rate, according to Montana Fish, Wildlife & Parks.